1. Field of the Invention
The present invention relates to a signal discrimination apparatus which identifies types of signals transmitted in a telephone communication line, e.g., determining whether a speech signal or a voiceband data signal is transmitted. In addition, the present invention relates to a signal transmitting apparatus for voiceband signals, wherein the signal discrimination apparatus is applied.
2. Description of the Related Art
A Digital Circuit Multiplication Equipment (called a DCME, hereinafter) is an example of a voiceband signal transmitting apparatus in which a signal discrimination apparatus is applied.
FIG. 55 shows an example configuration of the DCME. Reference numbers starting with a letter S such as S1, S2, . . . , in the drawings of this specification indicate signal lines. However, when indicating contents of signals transmitted in the signal lines is more helpful to understand the description than indicating the signal lines themselves, the contents of the signals transmitted in the signal lines are designated with the reference numbers starting with the letter S. Namely, in the following description, there are cases that the reference numbers starting with the letter S designate signal lines and are indicate contents of signals transmitted in the signal lines. Accordingly, the indications of contents can be differently stated depending upon how the signals are transmitted. There is a case of signals being stated in singular or plural.
In FIG. 55, an input signal S16, an activity detecting part 1, a signal discriminating part 2, a transmission controlling part 3, an encoding part 4, a facsimile signal demodulating part 5, a frame assembling part 6, a 2100 Hz detecting part 12, and a 1100 Hz detecting part 14 are shown. The input signal S16 of M channels is input into the DCME. The activity detecting part 1 judges whether each channel of the input signal S16 of M channels is active or silent, and outputs the judgement result as an activity detection result S1. The signal discriminating part 2 judges whether each channel of the input signal S16 of M channels is a speech signal or voiceband data signal, and outputs the judgement result as a signal discrimination result S2. The transmission controlling part 3 controls the encoding part 4, facsimile signal demodulating part 5 and the frame assembling part 6 based on the activity detection result S1 and the signal discrimination result S2, and outputs control information S3 to an opposite side apparatus (not shown, an apparatus having the same configuration as the apparatus of FIG. 55). The encoding part 4 includes encoders to the number of m, and encodes signals of m channels out of the input signal S16 of M channels based on designation of the transmission controlling part 3 in a low bit rate encoding fashion. The facsimile signal demodulating part 5 includes modems to the number of n, and demodulates signals of n channels out of the input signal S16 of M channels based on designation of the transmission controlling part 3. The frame assembling part 6 assigns encoded signals S4 of m channels and demodulated signals S5 of n channels to bits in a specified frame of the DCME, based on designation of the transmission controlling part 3. Then, the frame assembling part 6 outputs a generated DCME frame to the opposite side apparatus. The 2100 Hz detecting part 12 detects whether or not a tone of 2100 Hz exists in each channel of the input signal S16, and outputs a detection result S12. The 1100 Hz detecting part 14 detects whether or not a tone of 1100 Hz exists in each channel of the input signal S16, and outputs a detection result S14. Transmitting functions of the DCME are realized by the activity detecting part 1, signal discriminating part 2, transmission controlling part 3, encoding part 4, facsimile signal demodulating part 5, frame assembling part 6, 2100 Hz detecting part 12 and 1100 Hz detecting part 14.
In FIG. 55, a receiving controller 7, a frame disassembling part 8, a decoding part 9, a facsimile signal modulating part 10 and a pseudo background noise generating part 11 are also shown. The receiving controller 7 controls the decoding part 9, the facsimile signal modulating part 10, the pseudo background noise generating part 11 and the frame disassembling part 8 based on control information S20 received from the opposite side apparatus. The frame disassembling part 8 disassembles a DCME frame S19 received from the opposite side apparatus into an encoded signal S8 and a demodulated signal S21 depending upon designation of the receiving controller 7. Then, the frame disassembling part 8 outputs the encoded signal S8 and the demodulated signal S21 to the decoding part 9 and the facsimile signal modulating part 10. The decoding part 9 includes decoders to the number of m, and decodes the encoded signals S8 of m channels based on designation of the receiving controller 7. The facsimile signal modulating part 10 includes modems to the number of n, and modulates demodulated facsimile signals S21 of n channels based on designation of the receiving controller 7. The pseudo background noise generating part 11 generates a pseudo background noise based on designation of the receiving controller 7. Receiving functions of the DCME are realized by the receiving controller 7, frame disassembling part 8, decoding part 9, facsimile signal modulating part 10 and the pseudo background noise generating part 11.
Transmitting operation of the DCME will now be explained with reference to FIG. 55. It is known that there exists about 60 to 70% silent time out of the entire calling time in conversational speech signals such as a telephone communication, because one listens to another's speech without speaking. Therefore, it is possible to increase efficiency of transmission by connecting a communication line of m (m&lt;M) channels with signals of active speech channel out of input signals S16 of M channels and transmitting them. In the DCME, the activity detecting part 1 judges whether active or silent for each channel of the input signal S16 of M channels. The detection result S1 is transmitted to the transmission controlling part 3. The transmission controlling part 3 sends control information, relating to assignment of input channels to the encoders, to the encoding part 4 via a signal line S26, based on the detection result S1. The control information relating to the assignment tells that channels, wherein activity is recognized, in the input signal S16 of M channels should be assigned to the encoders to the number of m (m&lt;M) inside the encoding part 4 at a first priority.
The encoding part 4 encodes the input signals assigned to the encoders to the number of m in a low bit rate encoding fashion, and outputs the encoded signals S4. The encoding algorithm used in the encoding part 4 is for instance, an Adaptive Differential Pulse Code Modulation (called an ADPCM hereinafter) method prescribed in ITU-T (International Telecommunication Union--Telecommunication Standardization Sector) Recommendation G. 726. In the ADPCM method, it is possible to encode an input signal at a bit rate of 64 kbit/s resulting in a bit rate of 40 kbit/s, 32 kbit/s, 24 kbit/s or 16 kbit/s. In the case of applying the ADPCM method in the encoding part 4, it is desirable to select one of the above encoding bit rates based on the kind of input signal to be a speech signal or a voiceband data signal. When an input signal is a speech signal, it is better to select a lower encoding bit rate, within the range of maintaining good speech quality for communication, for efficiently utilizing the line. In this case, the encoding bit rate can be 32 kbit/s or lower. When an input signal is a voiceband data signal, it is necessary to select a higher encoding bit rate of 40 kbit/s in order not to generate a transmission error. Therefore, the signal discriminating part 2 which judges input signals to be speech signals or voiceband data signals is necessary for appropriately setting an encoding bit rate of the encoding part 4.
In the DCME shown in FIG. 55, the signal discriminating part 2 judges each channel of the input signal S16 to be a speech signal ("speech") or a voiceband data signal ("data"), and outputs the judgement result to the transmission controlling part 3. The transmission controlling part 3 sends control information relating to an encoding bit rate of the encoders, to the encoding part 4 via the signal line S26 based on the judgement result. The control information tells that the encoding bit rate of an encoder in the encoding part 4, to which a channel judged to be "data" out of the input signal S16 is assigned, should be at 40 kbit/s, and the encoding bit rate of an encoder in the encoding part 4, to which a channel judged to be "speech" out of the input signal S16 is assigned, should be at 32 kbit/s, 24 kbit/s or 16 kbit/s.
For instance, signals used for personal computer communication or facsimile communication are the voiceband data signals input into the DCME. It is possible to further increase the efficiency in utilizing the communication line, for example, by demodulating facsimile signals in the facsimile signal demodulating part 5 and transmitting them.
In the case of a facsimile signal input into the DCME being transmitted by demodulation, firstly the facsimile signal is input into a channel out of the input signal S16 of M channels and the signal discriminating part 2 judges a signal type for the channel. When the judgement result made by the signal discriminating part 2 is changed from "speech" to "data", the transmission controlling part 3 sends control information relating to an assignment of a modem to the input channel, to the facsimile signal demodulating part 5 via a signal line S18. The control information indicates that the facsimile signal demodulating part 5 should assign the input signal of the above channel to one modem out of modems to the number of n in the facsimile signal demodulating part 5.
Then, the signal of the above channel Is encoded at an encoding bit rate of 40 kbit/s in the encoding part 4 and transmitted to the opposite side apparatus. Simultaneously, in the facsimile signal demodulating part 5, the modem to which the facsimile signal of the above channel is assigned keeps monitoring presence of a signal modulated at a data rate of 300 bit/s, by the ITU-T Recommendation V. 21 Channel No. 2 modulation system used in the ITU-T Recommendation T. 30 protocol. When the signal modulated at the data rate of 300 bit/s is detected, it is recognized that the modulated signal is a facsimile call to be transmitted by demodulation. Then, the facsimile signal demodulating part 5 demodulates the facsimile signal input into the above channel, by the above modem. The facsimile signal demodulating part 5 outputs a demodulated signal S5 to the frame assembling part 6, and outputs control information, which indicates a beginning of transmission by demodulation of the facsimile call, to the transmission controlling part 3 via a signal line S17.
If the facsimile signal demodulating part 5 has not detected a signal modulated at 300 bit/s within a specific time since assigning the input channel to the modem was made based on the control information S18 from the transmission controlling part 3, the facsimile signal demodulating part 5 judges that the facsimile call can not be transmitted by demodulation. Then, the facsimile signal demodulating part 5 releases the assignment of the input channel to the modem. In addition, when a judgement result S2 of the signal discriminating part 2 changes to "speech" from "data", the transmission controlling part 3 judges that the facsimile call can not be transmitted by demodulation, and commands the facsimile signal demodulating part 5 to release the assignment of the input channel to the modem, through the signal line S18.
When the transmission controlling part 3 receives control information from the facsimile signal demodulating part 5 indicating that transmission by demodulation of the facsimile call is started, the transmission controlling part 3 commands the encoding part 4 to release the assignment of the input signal of the above channel to the encoder in the encoding part 4, through the signal line S26. At this moment, transmission of the facsimile call is switched to demodulation by the facsimile signal demodulating part 5, from the ADPCM encoding at 40 kbit/s.
The transmission controlling part 3 sends control information to the frame assembling part 6 through a signal line S27. This control information relates to assigning an encoded signal S4 of each channel output from the encoding part 4 to bits in a specified DCME frame, and relates to assigning a demodulated signal S5 of each channel output from the facsimile signal demodulating part 5 to the bits in the specified DCME frame. Based on the control information, the frame assembling part 6 assigns the encoded signal S4 of each channel and the demodulated signal S5 of each channel to the bits in the DCME frame. Then, the frame assembling part 6 sends data of the generated DCME frame to the opposite side apparatus through a signal line S6.
The transmission controlling part 3 sends control information relating to assigning an input signal of M channels to an encoder, relating to an encoding bit rate of ADPCM encoding, and relating to assigning an encoded signal and a demodulated signal to bits in the DCME frame, to the opposite side apparatus through the signal line S3.
The signal discriminating part 2 inputs the output S12 from the 2100 Hz detecting part 12, the output S14 from the 1100 Hz detecting part 14, and an output S24 from the receiving controller 7. Then, the signal discriminating part 2 resets its judgement result at "speech" or "data" based on these signals.
The 2100 Hz detecting part 12 detects whether or not a tone signal of 2100 Hz exists in the input signal S16, by performing a process such as a frequency analysis for the input signal S16. In the case of the tone signal of 2100 Hz existing, the 2100 Hz detecting part 12 outputs "1", and in the case of the tone signal of 2100 Hz not existing, the 2100 Hz detecting part 12 outputs "0", to the signal line S12 as a 2100 Hz detection result. The signal discriminating part 2 inputs the 2100 Hz detection result S12 and sets the judgement result S2 at "data" when the 2100 Hz tone is detected.
The 1100 Hz detecting part 14 detects whether or not a tone signal of 1100 Hz exists in the input signal S16, by performing a process such as a frequency analysis for the input signal S16. In the case of the tone signal of 1100 Hz existing, the 1100 Hz detecting part 14 outputs "1", and in the case of the tone signal of 1100 Hz not existing, the 1100 Hz detecting part 14 outputs "0", to the signal line S14 as a 1100 Hz detection result. The signal discriminating part 2 inputs the 1100 Hz detection result S14, and sets its judgement result S2 at "speech" when the 1100 Hz tone is detected.
The signal discriminating part 2 inputs the S24, showing a discrimination state of a receiving signal, from the receiving controller 7. When the signal discriminating part 2 detects a leading edge from "0" (speech) to "1" (data) of the discrimination state S24 of the receiving signal, the judgement result S2 is set at "data".
Receiving operation of the DCME will now be explained with reference to FIG. 55. The receiving controller 7 receives various control information output from a transmission controlling part in the opposite side apparatus, through a signal line S20. Depending upon the various control information, the receiving controller 7 outputs control information to the frame disassembling part 8, decoding part 9, facsimile signal modulating part 10 and the pseudo background noise generating part 11.
In addition, the receiving controller 7 judges whether the discrimination state of the receiving signal is "speech" or "data" based on the control information relating to the encoding bit rate of ADPCM encoding, received through the signal line S20. When the receiving controller 7 judges the state to be "speech", the output S24 is set at "0", and when the receiving controller 7 judges the state to be "data", the output S24 is set at "1". This output S24 is sent to the signal discriminating part 2 as stated above, and used for setting the judgement result S2 at "data".
The frame disassembling part 8 receives two kinds of control information from the receiving controller 7 through a signal line S25. One kind of control information relates to a bit assignment in the DCME frame received from the opposite side apparatus, for encoded data to be sent to the decoding part 9. The other kind of control information relates to a bit assignment in the DCME frame for demodulated data to be sent to the facsimile signal modulating part 10. Depending upon the two kinds of control information, the frame disassembling part 8 disassembles the DCME frame received from the opposite side apparatus through a signal line S19, and outputs an encoded signal S8 to the decoding part 9 and a demodulated signal S21 to the facsimile signal modulating part 10.
The decoding part 9 receives two kinds of control information from the receiving controller 7 through a signal line S23. One kind of control information relates to an encoding bit rate of each channel. The other kind of control information relates to an assignment of decoders to the number of m in the decoding part 9, to outputs of M channels from the DCME. Depending upon the two kinds of control information, the decoding part 9 assigns the encoded signal S8 received from the frame disassembling part 8 to one of the decoders to the number of m in the decoding part 9, decodes the assigned signal at an appropriate encoding bit rate, assigns the decoded signal of each channel to one of outputs of M channels of the DCME, and outputs the assigned signal to a signal line S9.
The facsimile signal modulating part 10 receives control information from the receiving controller 7 through a signal line S22. The control information relates to an assignment of modems to the number of n in the facsimile signal modulating part 10, to one of outputs of M channels from the DCME. Depending upon the control information, the facsimile signal modulating part 10 assigns the demodulated signal S21 received from the frame disassembling part 8 to one of the modems to the number of n in the facsimile signal modulating part 10, modulates the assigned signal at an appropriate data rate, assigns the modulated signal of each channel to one of outputs of M channels from the DCME, and outputs the assigned signal to a signal line S10.
The pseudo background noise generating part 11 receives control information from the receiving controller 7 through a signal line S7. Based on the control information, the pseudo background noise generating part 11 generates pseudo background noise for disconnected channels, not connected to the outputs of the decoding part 9 nor to the outputs of facsimile signal modulating part 10, out of the outputs of M channels from the DCME. Then, the pseudo background noise generating part 11 outputs the generated pseudo background noise to the disconnected channels through a signal line S11.
FIG. 56 shows an example of a signal discrimination apparatus used for the DCME. FIG. 56 shows a block diagram of a speech/data discrimination apparatus disclosed in Unexamined Japanese Patent Application 3-250961. A linear converter 101, a power judging part 102, a zero-crossing number judging part 103 and an AND circuit 104 are shown in FIG. 56. The linear converter converts an input PCM (Pulse Code Modulation) signal, being non-linearly quantized by an A-law and such, to a linearly quantized PCM signal.
Operation in FIG. 56 will now be described. A non-linearly quantized PCM signal S16 is converted to a linearly quantized PCM signal S101 in the linear converter 101. The linearly quantized PCM signal S101 is input into the power judging part 102 and the zero-crossing number judging part 103.
The power judging part 102 calculates a power ratio between blocks for the input linearly quantized PCM signal S101. Since signal level fluctuation of a voiceband data signal is smaller than that of a speech signal, a power ratio between blocks for the voiceband data signal is smaller than that for the speech signal. Based on this feature, the power judging part 102 judges whether an input signal is "speech" or "data". In the case of judging the input signal to be "speech", the power judging part 102 outputs "0", and in the case of judging it to be "data", the power judging part 102 outputs "1", to a signal line S102.
The zero-crossing number judging part 103 calculates a zero-crossing number (times of a signal crossing a zero level in a specific time) using the input linearly quantized PCM signal S101. Fluctuation of the zero-crossing number of the voiceband data signal is smaller than that of the speech signal, and a zero-crossing number distribution of the voiceband data signal is restricted within a specific range depending upon the modulation system of a modem. Based on this feature, the zero-crossing number judging part 103 judges whether the input signal is "speech" or "data". In the case of judging the input signal to be "speech", the zero-crossing number judging part 103 outputs "0", and in the case of judging it to be "data", the zero-crossing number judging part 103 outputs "1", to a signal line S103.
The AND circuit 104 calculates AND of an output S102 from the power judging part 102 and an output S103 from the zero-crossing number judging part 103. Then, the AND circuit 104 judges whether the input signal is a speech signal or a voiceband data signal, and outputs the judgement result to a signal line S2.
Namely, when a voiceband data signal is input into the speech/data discrimination apparatus, each of the power judging part 102 and the zero-crossing number judging part 103 judges the input signal to be "data", and sets each of the outputs S102 and S103 at "1". Calculating AND of the output S102 from the power judging part 102 and the output S103 from the zero-crossing number judging part 103, the output S2 of the speech/data discrimination apparatus becomes "1" ("data"). When a speech signal is input into the speech/data discrimination apparatus, one of the power judging part 102 and the zero-crossing number judging part 103 judges the input signal to be "speech", and outputs "0" as the output S102 or S103. Calculating AND of the output S102 from the power judging part 102 and the output S103 from the zero-crossing number judging part 103, the output S2 of the speech/data discrimination apparatus becomes "0" ("speech").
Operation of transmission of a facsimile signal by demodulation, in the case of the signal discrimination apparatus shown in FIG. 56 being provided as the signal discriminating part 2 in the DCME of FIG. 55, will now be explained.
FIG. 57 shows one example of the DCME being applied. In FIG. 57, a transmitting facsimile terminal is connected to one of input/output signal lines of M channels of a DCME located at a place, and a receiving facsimile terminal is connected to one of input/output signal lines of M channels of another DCME located at another place. An image data is transmitted from the transmitting facsimile terminal to the receiving facsimile terminal.
In the DCME to which the transmitting facsimile terminal is connected (called a transmitting DCME, hereinafter), an image data signal from the transmitting facsimile terminal is either encoded at 40 kbit/s or demodulated. Then, the transmitting DCME sends the encoded or demodulated signal to the opposite side DCME, that is the DCME to which the receiving facsimile terminal is connected (called a receiving DCME, hereinafter). The receiving DCME receives the encoded or demodulated signal from the transmitting DCME, decodes or modulates the receiving signal, and outputs the decoded or modulated signal to the receiving facsimile terminal.
FIG. 58 illustrates a signal state at each part inside the transmitting DCME and the receiving DCME in the case of a facsimile signal being input into the DCME. In FIG. 58, it is supposed that each initial state of outputs S2 of the signal discriminating parts 2 inside the transmitting DCME and the receiving DCME is "speech". Signals, transmitted and received between the transmitting facsimile terminal and the receiving facsimile terminal, in the beginning after a call connection, will be described with reference to FIG. 58.
First, the transmitting facsimile terminal outputs a tone of 1100 Hz, called a CNG, to show a non-speech terminal. The receiving facsimile terminal outputs a tone of 2100 Hz, called a CED. After the CED tone, the receiving facsimile terminal outputs a signal called a DIS in order to inform all the functions which belong to the receiving facsimile terminal, for the transmitting facsimile terminal. This DIS signal is modulated at a data rate of 300 bit/s by a method prescribed in Channel No. 2 of ITU-T Recommendation V. 21. The transmitting facsimile terminal receives the DIS signal, selects a function out of the functions designated by the DIS signal, and outputs a signal called a DCS in order to inform the selected function for the receiving facsimile terminal. This DCS signal is also modulated at a data rate of 300 bit/s by the method prescribed in Channel No. 2 of ITU-T Recommendation V. 21.
In the receiving DCME, when a CED tone (2100 Hz) is output from the receiving facsimile terminal, the output S12 of the 2100 Hz detecting part 12 is changed to "1" (detected) from "0" (not detected) by detecting the CED tone. The leading edge of the 2100 Hz detection result S12 from "0" (not detected) to "1" (detected) causes the judgement result S2 of the signal discriminating part 2 to set at a "data" state.
On changing to "data" from "speech" of the judgement result S2 of the signal discriminating part 2, assigning a modem is performed in the facsimile signal demodulating part 5 at the receiving DCME. Since then, this modem keeps monitoring the presence of the DIS signal modulated at a data rate of 300 bit/s by the method of V. 21 Channel No. 2.
When the receiving facsimile terminal outputs a DIS signal to the receiving DCME, the facsimile signal demodulating part 5 in the receiving DCME recognizes that a call from the transmission side is a facsimile call which can be transmitted by demodulation. Since then, transmission of the facsimile call is switched to a transmission by demodulation from the transmission by ADPCM encoding at 40 kbit/s.
In the transmitting DCME, when the receiving controller 7 recognizes a discrimination state change to "data" from "speech" of a receiving signal, based on control information relating to an encoding bit rate of ADPCM encoding received through the signal line S20, the receiving controller 7 changes the output S24 to "data" from "speech". The above discrimination state change of the receiving signal to "data" from "speech" results from the CED tone (2100 Hz) output from the receiving facsimile terminal. The signal discriminating part 2 sets its output S2 at "data" based on the output S24 from the receiving controller 7.
On changing to "data" from "speech" of the judgement result S2 of the signal discriminating part 2, assigning a modem is performed in the facsimile signal demodulating part 5 at the transmitting DCME. Since then, this modem keeps monitoring the presence of the DCS signal modulated at a data rate of 300 bit/s by the method of V. 21 Channel No. 2.
When the transmitting facsimile terminal outputs a DCS signal to the transmitting DCME, the facsimile signal demodulating part 5 in the transmitting DCME recognizes that a call from the receiving side is a facsimile call which can be transmitted by demodulation. Since then, transmission of the facsimile call is switched to a transmission by demodulation from the transmission by ADPCM encoding at 40 kbit/s.
FIG. 59 shows another example of operation in the case of a facsimile signal being input into the DCME shown in FIG. 55. FIG. 59 shows a signal state of each part on supposition that each initial state of the outputs S2 of the signal discriminating parts 2 in the transmitting DCME and receiving DCME is "data".
In the receiving DCME, when a CED tone (2100 Hz) is output from the receiving facsimile terminal, the output S12 of the 2100 Hz detecting part 12 is changed to "1" (detected) from "0" (not detected) by detecting the CED tone. However, since the initial state of the judgement result S2 of the signal discriminating part 2 is "data", the judgement result S2 of the signal discriminating part 2 is not effected by the leading edge to "1" (detected) from "0" (not detected) of the 2100 Hz detection result S12. Namely, the judgement result S2 of the signal discriminating part 2 keeps "data" state.
Since the judgement result S2 of the signal discriminating part 2 does not change to "data" from "speech", assigning a modem is not performed in the facsimile signal demodulating part 5 at the receiving DCME. Accordingly, this facsimile call can not be transmitted by demodulation but can be transmitted by ADPCM encoding.
In the transmitting DCME, when a CNG tone (1100 Hz) is output from the transmitting facsimile terminal, the output S14 of the 1100 Hz detecting part 14 is changed to "1" (detected) from "0" (not detected) by detecting the CNG tone. The judgement result S2 of the signal discriminating part 2 is reset at "speech" from "data" based on the above change.
After then, even when a CED tone (2100 Hz) is output from the receiving facsimile terminal, the discrimination state S24 of the receiving signal keeps a state of "data" in the transmitting DCME. Since there is no change to "data" from "speech", the output S2 of the signal discriminating part 2 maintains "speech" state without being effected to be "data" by the output S24 of the receiving controller 7.
Then, when the transmitting facsimile terminal outputs a DCS signal to the transmitting DCME, the signal discriminating part 2 in the transmitting DCME judges the DCS signal to be "data". However, there exists a delay-time between the instant of DCS signal being input and the instant of the DCS signal being judged to be "data". In addition, since the discrimination state of the DCS signal at the first moment of being input is "speech", assigning a modem is not performed in the facsimile signal demodulating part 5. Therefore, this facsimile signal can not be transmitted by demodulation but can be transmitted by ADPCM encoding.
Namely, the following requirements are needed for the operation of the signal discriminating part 2 so that the facsimile signal can be transmitted by demodulation through the DCME. One requirement for the operation is that the judgement result S2 should change to "data" from "speech" before a signal modulated at a data rate of 300 bit/s is input. The other requirement for the operation is that the judgement result S2 should be "data" at the beginning of inputting the signal modulated at the data rate of 300 bit/s.
For the purpose of making the change to "data" from "speech" of the judgement result S2 before the signal modulated at the data rate of 300 bit/s is input, it is necessary that the signal discrimination state should be reset at "speech" at the disconnection of a previous call or at the connection of a new call. One method of resetting the judgement result S2 of the signal discriminating part 2 at "speech" is that the facsimile signal demodulating part 5 monitors a protocol for receiving/transmitting facsimile signals, detects an end of a procedure for receiving/transmitting facsimile signals, and outputs a reset signal which tells the signal discriminating part 2 to compulsorily reset its discrimination state at "speech".
However, the above method has the following problem. When a signal, except a facsimile call, such as a modem signal used for personal computer communication is input into the DCME, the facsimile signal demodulating part 5 judges that the call can not be transmitted by demodulation, since the facsimile signal demodulating part 5 does not detect a signal modulated at the data rate of 300 bit/s within a specific time. Therefore, the facsimile signal demodulating part 5 releases the modem assignment. As the facsimile signal demodulating part 5 does not output and is not going to output the reset signal which tells the signal discriminating part 2 to compulsorily reset its discrimination state at "speech", the discrimination state still keeps "data" even when the call for personal computer communication is finished. In the case of another facsimile call being established at this moment, there exists a problem that the facsimile call can not be transmitted by demodulation, because the change to "data" from "speech" of the signal discrimination state does not occur and the modem assignment is not performed for the facsimile call.
There is another method of resetting the judgement result S2 of the signal discrimination state at "speech" at the disconnection of a previous call or at the connection of a new call. The method is that, using a signal discrimination apparatus as shown in FIG. 60, the signal discrimination state can be reset at "data" at the disconnection of a previous call or at the connection of a new call by receiving call control information from the outside. In FIG. 60, the linear converter 101, the power judging part 102, and the zero-crossing number judging part 103 are the same as those shown in FIG. 56. SS and SR are signalling signals in a channel associated signalling system. SS is a signalling signal from a local exchange located at the near side (same side as the present apparatus) and SR is from a remote exchange located at the opposite side (same side as the opposite apparatus). A reset signal generating part 106 inputs the SS and the SR, and generates a reset signal. A discrimination result outputting part 105 judges whether an input signal is "speech" or "data" based on each output from the power judging part 102, zero-crossing number judging part 103 and the reset signal generating part 106.
Now, operation of the signal discrimination apparatus will be stated. The operations of the linear converter 101, power judging part 102 and the zero-crossing number judging part 103 are the same as those described with reference to FIG. 56.
The reset signal generating part 106 inputs the signalling signal SS from the local exchange and the signalling signal SR from the remote exchange. Then, the reset signal generating part 106 detects a call connection based on the state of the signalling signals SS and SR. When the call connection is detected, the reset signal generating part 106 outputs a reset signal as S106.
The reset signal S106 is input into the power judging part 102, zero-crossing number judging part 103 and the discrimination result outputting part 105.
When the reset signal generating part 106 does not output a reset signal to the signal line S106, the discrimination result outputting part 105 applies a judgement result based on an output S102 from the power judging part 102 and an output S103 from the zero-crossing number judging part 103. Namely, the discrimination result outputting part 105 calculates AND of the output S102 from the power judging part 102 and the output S103 from the zero-crossing number judging part 103, and outputs a judgement result of the input signal being "speech" or "data", to the signal line S2.
When the reset signal generating part 106 outputs a reset signal to the signal line S106, the discrimination result outputting part 105 resets its output S2 at "0" ("speech") regardless of the output S102 from the power judging part 102 and the output S103 from the zero-crossing number judging part 103.
While the reset signal generating part 106 outputs a reset signal to the signal line S106, the power judging part 102 and the zero-crossing number judging part 103 reset their internal states in order that the outputs S102 and S103 can be "0" ("speech").
By applying the above configuration, it is possible to make an initial state of an output for signal discrimination at the beginning of call be "speech", because the reset signal generating part 106 detects a call connection based on the state of the signalling signal and resets the discrimination state at "speech" at the moment of detecting the call connection.
However, the signal discrimination apparatus shown in FIG. 60 is provided on condition that the signalling signal SS from a local exchange and the signalling signal SR from a remote exchange are supplied as call control information. Accordingly, there is a problem that the signal discrimination apparatus can not be used in a system where such call control information is not provided.
The present invention is contrived so as to solve the above problems. It is an object of the present invention to provide a signal discrimination apparatus in which a discrimination state can be reset at "speech" at the beginning of a facsimile call even when call control information can not be obtained.
In addition, it is another object of the present invention to provide a signal transmitting apparatus for voiceband signals, wherein a facsimile signal can be transmitted by demodulation even when call control information can not be obtained.